1. Field of the Invention
The present invention relates to an ink jet recording apparatus and its recording method for recording an image with a recording head having a plurality of orifices.
2. Description of the Prior Art
A multi-droplet method is a recording method in which a plurality of ink droplets ejected from a single orifice is projected substantially onto an identical position on the recording sheet to form a pixel, and the number of ejected ink droplets of the pixel is changed to express halftoning (gray-scale) of an recorded image. The multi-droplet method is an effective for expressing a halftoning, particularly in a recording system in which ink droplets are ejected by a bubble generated in an ink fluid by thermal energy, and in which it is difficult to change the size of each ink droplet to a large extent.
As a single pixel is formed by a plurality of ink droplets ejected from a single orifice in the multi-droplet recording method, in case that the amount of ejected ink droplets varies from orifice to orifice, a density level (gray level) in the recorded image cannot maintain to be uniform and the recorded image may contain strip-noises or shadings (or bandings).
In order to prevent those problems, in prior art recording head, the manufacturing process of the recording heads must be controlled precisely by a software-based system for reducing the variation of the amount of ejected ink droplets from orifice to orifice. As a result, there are many problems including that a manufacturing cost may increase.
To solve the problems described above, an assignee of the present invention proposed a recording method in which a single pixel is formed by a plurality of ink droplets ejected from a plurality of orifices. That is, in the proposed method, so-called a multi-scan recording method, each of ejections of ink droplets for forming a single pixel from each of a plurality of orifices is performed at respective scanning of a recording head. The method will be appreciated that the amount variation of ejected ink droplets can be distributed uniformly from orifice to orifice.
FIG. 1 is a schematic illustration illustrating the multi-scan recording method.
In FIG. 1, reference numeral 801 denotes a recording head having a plurality of orifices, in this case, the number of orifices is 128. The recording head 801 forms a recorded image by ejecting ink droplets during scanning thereof in the horizontal direction in the figure from left to right. 801A is a position of the first scanning of the recording head 801 in relative to a recording medium, and 801B, 801C and 801D are positions corresponding to the subsequent scannings of the recording head, respectively. Reference numeral 802 denotes a pixel. The number having a symbol # as its prefix is an identification number specific to each of 128 orifices. In the above structure, a density level of the specified pixel 802 at a pixel position (x, y) is expressed by selecting ejections and non-ejections from orifices No. 100, No. 68, No. 36 and No. 4 at each of four scannings 801A, 801B, 801C and 801D of the recording head 801. This means that, for example, if all the orifices do not eject ink droplets, the density level of the specified pixel 802 at (x, y) is minimized, and if all the orifices eject ink droplets, the density level is maximized. The intermediate density level between its minimum and maximum is expressed by a combination of selective ejections and non-ejections of ink droplets from orifices. In this prior art recording methods, what is explained is a recording method in which a pixel having 5 levels of the density level is formed and the number of ink ejections between 0 and 4 to each pixel is obtained by pseudo-halftoning processing such as an error diffusion method and a dither method and is stored in a memory, and in which data of the number in the memory is read out at every time of scannings, and thus, a pixel is formed by driving the recording head in responsive to this read-out data.
The ejection timing for forming the pixel 802 by each of orifices No. 100, No. 68, No. 36 and No. 4 in the scannings 801A, 801B, 801C and 801D of the recording head 801 is established by pulses from an encoder measuring the relative distance between the recording head 801 and the recording medium. After each of scannings 801A, 801B, 801C and 801D, the recording medium is transported with respect to the recording head in the direction vertical to the scanning direction by 32 times of the distance between adjacent orifices, that is, 32 pitches obtained by 128 divided by 4. By scanning four times the recording head, an image corresponding to 128 orifices can be recorded.
In the case of forming a single pixel with a single ink droplet, by selecting appropriate one of a plurality of orifices, the amount variation of ejected ink fluids from orifice to orifice can be reduced.
FIG. 2 is a schematic illustration showing an arbitrary part of an array of pixels arranged in the scanning direction of the recording head. Each pixel in the array is formed by ink droplets ejected from orifices No. 100, No. 68, No. 36 and No. 4 in the respective scanning 801A, 801B, 801C and 801D as shown in FIG. 1.
In FIG. 2, reference numerals 901 to 909 denote pixels, each number written in the pixels 901 to 909 represents the number of ejections to each pixel. The identification number of the orifice used for forming the pixel and the scanning number at which the orifice ejects an ink droplet are written below each of the corresponding pixels. For example, the number of ejections to the pixel 904 is one and the orifice No. 100 ejects an ink droplet in the scanning 801A for forming the pixel 904. And also, the number of ejections to the pixel 905 is two, and the orifices No. 100 and No. 68 eject ink droplets in the scannings 801A and 801B, respectively, for forming the pixel 905.
In FIG. 2, in the case that the number of ejections is one as to the pixels 901, 902 and 904, an ink droplet is always ejected from the orifice No. 100 in the scanning 801A. In the case that the number of ejections is two as to the pixels 905, 906 and 909, one ink droplet is always ejected from the orifice No. 100 in the scanning 801A and another ink droplet is always ejected from the orifice No. 68 in the scanning 801B. Similarly, in the case that the number of ejections is three as to the pixel 908, the orifice No. 100 is used in the scanning 801A, the orifice No. 68 is used in the scanning 801B and the orifice No. 36 is used in the scanning 801C, respectively. In the case that the number of ejections is four, all the orifices are always used in their corresponding scanning for forming a pixel. However, in the case of forming a pixel in the above manner, if there occur, for example, in the orifice No. 100 always used, ejection failures such as disabled ejections, the less amount of ejected ink droplet, and further, ejection of ink droplet in unexpected directions, the recorded images may contain strip-noises or shadings. So far, even if a single pixel is formed by a plurality of ink droplets ejected from a plurality of orifices, there may be a case that high-quality recorded images cannot obtained and recording images is spent on too long time. In addition, a specific orifice, for example, the orifice No. 100, is used too often and other orifices are not used so frequently, and hence, there may be such problems that every orifice is not used uniformly and the lifetime of the recording head may be shorten.
This problem arises similarly in case of forming a single pixel with a single ink droplet.